1. Field of the Invention
The present invention generally relates to magnetic resonance imaging (MRI) utilizing nuclear magnetic resonance (NMR) phenomena. More particularly, this invention is directed to a novel MRI magnet flux path for use in an MRI system.
2. Description of the Prior Art
A detailed discussion of the existing prior art with respect to MRI magnetic flux path and the upper and lower poles and their interconnection to provide the necessary flux return path is set forth in the above prior art patents. As noted in the prior art, a great variety of MRI systems are now commercially available and they vary greatly in design criteria as well as purchase, installation and operating costs.
As discussed in U.S. Pat. No. 5,250,901 (hereinafter the '901 patent"), and as shown in FIG. 4 thereof, the cost of an MRI magnet generally increases as the required magnetic field strength increases if a permanent magnet is used to provide the B.sub.o field. However, if a traditional superconductive magnet is used, the higher initial cost is overcome at the higher field strengths. For the purposes of this discussion, a "low" strength field is one in the range of 0.00 T up to 0.3 T, a medium magnetic field strength is from 0.3 T to about 1.0 T and a high strength field is one having a field strength greater than 1.0 T and, in particular, field strengths of 1.50 T or greater.
Conventional wisdom is that the signal to noise ratio (S/N) of an MRI system is proportional to the magnetic strength of the B.sub.o field, but various improved signal processing procedures have permitted high S/N with relatively low strength fields. The Assignee of the present invention makes a series of highly successful permanent magnet MRI systems which are characterized by a low field strength. In such systems, the static magnetic B.sub.o field is created by a permanent magnet and the magnetic flux return path between pole faces comprises a series of vertically extending yokes which support and space apart the upper and lower poles. These vertically extending yokes are the only structure surrounding a patient located in the patient imaging volume.
One advantage of the above such systems, as discussed in the previous patents, is that the expense of maintaining a superconductive or resistive electromagnet is avoided. A further advantage is to alleviate to a large extent any patient discomfort caused by a claustrophobic reaction to conventional prior art MRI magnets using air as the magnetic circuit medium. An additional advantage of such open architecture is that easy access to the patient is provided during MRI scanning. This access facilitates the implementation of surgical procedures and/or patient diagnosis during MRI scanning.
Even with the advent of more sophisticated signal processing to provide improved S/N with the low field MRI systems, it is desirable to provide a medium field power system which, when combined with the advanced signal processing, provides a resolution equal to or greater than previous high power MRI systems. Unfortunately, the cost and weight of permanent magnets renders it impractical to use permanent magnets to provide the B.sub.o field with a field strength above 0.3 T. Consequently, the inductry has generally turned to superconducting materials to form high current, high magnetic field electric magnets with suitable iron cores to conduct the magnetic flux through the patient imaging volume. Because relatively high critical temperature (T.sub.c) (77.degree. k or higher) superconducting materials may become available and higher temperature superconducting materials are being developed, the use of the term "superconducting material" in this application encompasses all superconducting materials currently known and those developed in the future, regardless of their critical temperatures.
Existing superconductor electromagnetics are generally located and associated with both the upper and lower poles so as to generate a homogeneous magnetic field in the air gap between the pole faces, especially in the region of the patient imaging volume. Unfortunately, a consequence of the different locations of the two superconducting electromagnetics is the requirement of two separate cryostats, one for each electromagnet, along with the necessary linkage structure for linking the two electromagnet coils together and their associated linkage cryostat. Accordingly, as disclosed in the '901 patent, discussed above, an improvement in operating efficiency can be made if only a single cryostat is used to house all windings of the superconducting electromagnet (see FIG. 7 in the '901). However, the utilization of only a single cryostat causes some difficulties with respect to the uniformity of the magnetic field through the patient imaging volume.
One critical tradeoff requirement of MRI systems is the spacing between pole faces. In order to maintain the highest possible magnetic field strength, the pole faces should be very close together. However, the spacing between the pole faces must be sufficient for a patient or the object being imaged to be comfortably inserted into the field. This air gap distance between pole faces is conventionally on the order of 60 cm. In order to maintain a highly permeable flux path and to avoid saturation (and consequent reduction in the magnetic flux across the air gap), a rather substantial flux return path is provided between the pole faces.
Where a single cryostat embodiment is shown in the prior art (see FIG. 7 of the '901 patent), a substantial flux return path is provided so as to avoid saturation while at the same time maintaining a relatively high strength field. Unfortunately, and especially so with respect to a single superconducting electromagnet as shown in the prior art, the static magnetic B.sub.o field in the patient imaging volume can be relatively non-homogeneous and requires well-known shim coils or shim irons (a resistive coil or circle of iron placed adjacent the superconducting magnet to modify the field between the upper and lower poles).
To avoid the need for separate cryostats, the shim coils are generally resistive electromagnets, rather than superconducting electromagnets, further adding to the electrical power requirements of the system and generating additional heat which must then be removed from the system in order to avoid discomfort to the patient or damage to the operation of the magnet.